Image-forming apparatuses that use electrophotographic technology, e.g., copiers and printers, are currently in wide use. The image-forming method here comprises an electrostatic latent image-forming step, in which an electrostatic latent image is formed on a charged electrostatic latent image-bearing member; a development step, in which this electrostatic latent image is electrostatically developed by toner being carried on a toner-carrying member; a transfer step, in which the toner image on the electrostatic latent image-bearing member is transferred to a transfer material; and a fixing step, in which this toner image is fixed on a recording medium, e.g., paper, by the application of, e.g., heat or pressure.
Image-forming apparatuses that use electrophotographic technology, e.g., copiers and printers, have in recent years been experiencing increasing diversification in their intended applications and use environments. In combination with this, there is also strong demand for additional increases in speed and for an even longer service life.
However, when the apparatus is sped up, there may not be enough time for the toner on the toner-carrying member to be adequately charged, and as a consequence uniform charging of the toner may be impaired. This phenomenon becomes more significant during long-term use, during which the toner composition becomes increasingly nonuniform.
Various problems, such as a reduction in the development efficiency and a reduction in the transfer efficiency, are produced when the charge distribution on the toner becomes nonuniform. One of these problems is a type of phenomenon known as electrostatic offset.
Electrostatic offset is a phenomenon in which, prior to the unfixed image being fixed by the fixing apparatus, toner on the unfixed image flies to and attaches to locations on the fixing apparatus that are in contact with the unfixed image, as a consequence of which toner is fixed on the recording medium in a manner unrelated to the locations defined by the electrostatic latent image and image defects are thereby produced. Normally, the toner is electrostatically attached on the unfixed image and is also not electrically drawn to the contact locations of the fixing apparatus. However, toner with a charge opposite from the normal charge is sometimes produced when the charge distribution on the toner becomes nonuniform. An electrostatic force is produced between the oppositely charged toner and the contact locations of the fixing apparatus, and as a result the oppositely charged toner randomly flies over the contact regions and image defects are then ultimately produced. This phenomenon has been more significant in high-temperature, high-humidity environments, where toner charge leakage is induced and the production of charging defects is thereby facilitated.
Maintaining a uniform charge distribution is crucial for inhibiting electrostatic offset; however, even if the process of imparting charge to the toner on the toner-carrying member can somehow be strengthened, the charge is subsequently reduced in the transfer step and on the recording medium and it is thus quite difficult to completely prevent the appearance of nonuniformity. Due to this, there have been limits on the approach of completely suppressing the occurrence of oppositely charged toner.
Another approach that has been contemplated here is the inhibition of flight of the oppositely charged toner by causing a semi-melting of the toner on the unfixed image in the neighborhood of the fixing device in order to bring about toner unification or coalescence.
Specifically, numerous toners exist that contain a crystalline polyester that rapidly melts in response to heating of the toner (Patent Documents 1 to 4); however, none of these have achieved a satisfactory coalescence of the toner on the unfixed image and they have not been a satisfactory countermeasure to electrostatic offset. However, additional increments in the amount of a simple crystalline polyester end up producing various problems with, e.g., the charging performance and environmental stability.
Thus, there has been demand for a toner that can suppress electrostatic offset based on an approach from a novel perspective.